Advanced Theater Transport

In the long term (FY11-21) the Air Force plans to begin acquisition process for the Advanced Theater Transport to replace C-130s as they retire. The Advanced Theater Transport concept would allow delivery and extraction of medium-weight fighting systems and their support at a much wider range of airland terminals than is now available. Among the designs being considered are tilt-rotor and inclining-wing concepts that potentially could get 30-plus ton loads in and out of runways as short as 500 feet.

This long term replacement aircraft for the C-130E/H includes enhanced reliability, maintainability, and availability; advanced cargo handling features; super short takeoff and landing capability; oversized/outsized cargo capability; high speed/low level airdrop capability; articulated cargo ramp; high lift systems with externally blown flaps; fly-by-wire capability; off-the-shelf derivative engines; cross-shafted propellers and rotors; off runway landing gear; advanced cockpit design with autonomous landing capability and onboard mission planning. Survivability features include IR suppression, reconfigurable flight controls, damage tolerance, and California Bearing Ratio hardening. In addition, it must have at least the same capabilities as C-130J.

The ATT tactical mission is a 1,000 km radius, Hi-Low-Low-High profile with SSTOL landing and takeoff at mid-point. The mid-point and the 100 nm on either side of mid-point are at 4,000 ft MSL (above mean sea level) on a 95 ºF day. The rest of the mission is flown at optimum altitude on a standard day. A 30 ton payload is carried both out and back. The ATT has full 3.0g combat maneuverability and low altitude dash speed capability under these conditions. The initial takeoff is from a long runway and is not a limiting factor. The mid-point takeoff and landing performance plot is based on the ATT tactical mission profile. The mid-point gross weight includes enough fuel to fly a 1,000 km return leg plus a fuel reserve. The runway length (ground roll) required to carry a 30 ton payload is just over 750 ft for landing and just under 750 ft for takeoff. The total landing zone length (including runway) for landing over a 50 ft obstacle is just over 1,250 ft for a 30 ton payload. These numbers are based on wartime "assault rules" which allow the ATT to roll beyond the end of the runway after an engine failure.

The payload range plot is based on a deployment mission profile. The entire mission is flown at optimum altitude on a standard day, without a mid-point landing. This is more fuel efficient than the tactical mission profile. Hence the range for a 30 ton payload (1,300 nm or 2,400 km), is greater than the 2,000 km needed to fly a 1,000 km radius tactical mission. ATT can deploy with a 40 ton payload (e.g., two 20 ton FCS) at a 2.25g load factor. Operation at the high Maximum TakeOff Gross Weight (MTOGW) allowed by a 2.25g load factor is restricted to emergency wartime situations due to structural limitations on turbulence penetration and maneuverability. This is a caution area in the flight manual. ATT can carry a 36 ton payload at its normal 2.5 g load factor. Normal operation has limitations on maneuverability and low altitude dash speed compared to combat operation.

Two concepts in direct competition with each other are the Advanced Theater Transport (ATT) aircraft designs of Boeing and Lockheed.

Boeing's early tilt wing concept, developed in the late 1990s was a tail-less design called the Super-Frog. The performance was computed using Boeing's theoretical powered lift aerodynamics methods, which calculated lift and drag based on thrust coefficient and geometric parameters. Trimmed flight analysis was limited with these methods. As detailed design work was commenced in 2000, other databases and methodologies were investigated to improve pitching moment predictions and validate Boeing's empirical high lift methods. It became evident that the last significant amount of research on tilt-wing propeller designs was concluded in the early 1970s. The USAF Light Intratheater Transport (LIT) studies in 1968 were a valuable source for tilt-wing design methodology.

In May 2000 Boeing unveiled a Advanced Theater Transport "Super Frog" concept, a four-engine, tailless STOL tiltwing that can carry 80,000 pounds, cruise up to 410 kts, and land at speeds as low as 36 kts. The Boeing ATT concept is a four-engine, no tail, tilt-wing aircraft capable of landing and taking off from strips as short as 600 feet. Boeing plans to design the aircraft so that lift will be generated by the wing itself at all times. While this will rule out VTOL capabilities, it will allow the new airlifter to operate with a 100,000 pound payload. It could carry up to 100 troops or 11 cargo pallets-as well as ground vehicles such as Humvee trucks. Examples of possible cargo loads include: one Army MRLS rocket launcher with missiles, two Humvees and 40 troops; or five Humvees and 32 troops; or one 5-ton truck, one 8-ton truck and 40 troops.

The ATT would carry twice the load and operate in about three-quarters of the C-130J's required ramp space. The wing in the ATT tilts upwards in a 45-degree angle, leading to a propulsion requirement for 50,000 pounds of thrust, compared to 150,000 pounds of thrust for vertical lift. The aircraft employs high lift devices such as externally blown flaps, similar to those used on the C-17. Initial analysis on this concept concluded that with the fly-by-wire flight control system and design of the body, the control effectiveness at slow speed offered by an elevator is marginal. For this reason and in the interest of weight savings, the present design calls for a tailless configuration. The wingspan and length of the aircraft are less than that of a C-130, while the fuselage is almost twice as wide.

Powered by four (eight-bladed) turboprop engines, the aircraft features a tailless fuselage with a forward swept tilt-wing arrangement. This unique configuration provides for enhanced performance capabilities for landing and takeoff within 750 feet carrying 80,000 pounds of payload in very austere operating conditions. The tilt-wing design and computerized flight controls allow for steep ingress and egress angles, increasing flexibility in hostile areas, as well as its ability to quickly establish and replenish forces in forward operating areas.

With a typical payload of 80,000 pounds, the ATT can fly more than 3,000 nautical miles. It can also use its 115,000 pounds of wing fuel to airlift large quantities of fuel to advancing operating forces. Such impressive strategic and theater lift flexibility would make the ATT concept a valuable asset for theater lift and augmenting vertical lift assets in forward areas. ATT's fuselage cross-section is similar to the C-17 configuration and will be able to accommodate two of the U.S.Army's 20-ton advanced Future Combat System vehicles, as well as additional outsized loads including Bradley vehicles, Patriot missile systems, MLRS (Multiple Launch Rocket System), HEMTT (Heavy Expanded Mobile Tactical Truck) and PLS (Palletized Load System). The ATT's autonomous cargo handling System increases cargo through-put capability and eliminates or minimizes the need for the advance placement of material handling equipment at forward bases.

This early study of a wing-mounted turboprop propulsion installation showed that a propeller powered, tilt wing design offers outstanding SSTOL performance. Still in the early design process, the ATT program has had a number of configuration updates since 2000. A 2003 configuration had four highly loaded large diameter propellers mounted on a tilt-wing with flaps to obtain the required high lift. The outboard section of the wing is swept forward to improve the pitch augmentation potential of the wing control surfaces, which minimizes the aft stabilizer area. The inboard and outboard propellers have an overlap of 15% of one propeller diameter.

The Lockheed Circulation Control Wing concept is a subset of boundary layer control. In order to provide continued control at low speeds, the circulation control is basically designed to prevent the wing from stalling. This is done by introducing a high-energy jet onto the upper, trailing edge region of a blunt trailing edged airfoil (occasionally leading-edge blowing is also used). This method to produce lift at low speed, combined with other advanced high lift devices and the lift efficiency of a blended wing-body design, is the cornerstone of the Lockheed ATT strategy for SSTOL. As with the Boeing concept, the concept has not been funded to allow for a prototype.

Analysis showed that to operate with the thirty ton payload initially set by the Army, the aircraft can land in the required distance, but would require nearly 1,200 feet for takeoff. The 750-foot runway would allow payloads for takeoff of up to 20,000 pounds.

In accordance with the Defense Planning Guidance, in October 2002 the Army submitted its Transformation Roadmap, which outlined the Army's Transformation strategy and detailed how Army Transformation supports sustained progress toward the attainment of the operational goals for Transformation stated in the 2001 Quadrennial Defense Review. As directed by the Secretary of Defense's Transformation Planning Guidance, the Army presente its first annual update to the Army Transformation Roadmap in 2003. This document defined a Super-Short Takeoff and Landing (SSTOL) as an aircraft with the ability to carry two FCS platforms 3,500 miles. It can land on 750 feet of road or field in the joint area of operations. This represented a considerable increase in the range requirement relative to the nominal ATT tactical mission, which was a 1,000 km radius, Hi-Low-Low-High profile with SSTOL landing and takeoff at mid-point with a 40 ton payload (e.g., two 20 ton FCS).